In recent years, infusion apparatus has been developed which can be implanted in the body and remain there for a prolonged period. The apparatus can be refilled with infusate without having to remove the apparatus from the patient's body by injecting additional infusate through a penetrable septum in the apparatus wall located directly under the patient's skin. Examples of infusion apparatus of this general type are disclosed in U.S. Pat. Nos. 3,731,681 and 3,951,147.
In the treatment of some patients such as those afflicted with diabetes, the amount of medication such as insulin infused per unit of time should be adjusted at certain time intervals. This is because the requirements of the patient usually fluctuate during the day, such fluctuations being caused, for example, by the ingestion of food. Some prior implantable infusion apparatus provide this flexibility, examples being shown in U.S. Pat. Nos. 3,894,538 and 4,077,405.
It has also been proposed to program the daily administration of medication such as insulin. In such apparatus, the piston of an infusate injector is moved by a motor drive in accordance with said program in such a way that the desired daily dose is achieved while accounting for fluctuations in the patient's glucose level, temperature and the ambient pressure to which the patient is subjected. See U.S. Pat. No. 4,003,379 and the references cited therein.
The aforesaid implantable infusion pumps are, however, disadvantaged in one or another of the following respects. Some require an excessive number of mechanical components. This renders it impractical to manufacture such devices with the required small size to enable them to be implanted in the patient's body without undue discomfort to the patient. Some of the prior devices control infusate flow by means of flow restrictions or controllable valves. If the infusate is a type of fluid such as insulin whose molecules agglomerate when they are disturbed, the resultant squeezing of the infusate through such restrictions and valving damages the molecules and also causes clogging of those restricted passages.
Further, some of the conventional infusion devices of this general type have an excessive energy requirement either to develop the necessary pressure to dispense the infusate into the patient's body or to regulate that flow to provide the desired dosages. If the energy requirement is supplied by a battery, that battery has to be replaced or recharged relatively often, requiring, at the very least, penetration of the patient's skin, giving rise to the possibility of infection.
Some prior pumping apparatus of this type only dispense infusate to the patient intermittently or periodically which is disadvantageous in some instances. For example, tests have shown that diabetics should receive a basal dose of insulin which is continuous and the basal dose supplemented by so-called bolus doses at certain times of the day, such as at mealtimes. The difference in the basal and bolus flow rates may be several orders of magnitude and it is quite difficult to achieve proper flow control over that entire range of flow rates. The device disclosed in U.S. Pat. No. 4,140,122 does have the advantage of achieving a continuous dosing of infusate even at very small flow rates. However, that continuous feeding or injection of medication also requires a continuous generation of pressure and consequently a constantly higher requirement of input energy for the electrodes which control infusate flow. Furthermore, that requirement increases substantially when the rate of medication infeed increases.
In general, the prior comparable devices of which we are aware do not take into account all of the physiological concepts and clinical factors involved in the various therapies, particularly diabetes therapy, in connection with which such implantable devices are used. In general, such implantable infusion apparatus should have the ability to:
1. deliver infusate such as insulin at a wide range of bolus and basal flow rates, thereby accommodating the needs of the majority of the afflicted population; PA1 2. tailor the infusate delivery scheme to individual patients without having to physically modify or alter the pump components; PA1 3. control the required infusion rates to an extreme accuracy over long time intervals; PA1 4. limit induced shear forces in the infusate, thereby maintaining its solution integrity; PA1 5. allow external programming to permit patient/physician interaction; this allows the physician to (a) match the pump's flow rate parameters to each patient's requirement, (b) monitor the device for signs of past or impending failure, (c) retrieve information on past pump operations (e.g. average daily basal and bolus flow rates, total infusion delivery to date, etc.), and (d) allow the patient to set meal characteristics so that the bolus dose can be tailored for each meal instead of being set at a predetermined average dose level; and PA1 6. exhibit long life capabilities on the order of ten years to be able to justify the cost of the device, the cost of the surgical implant and the medical risks undertaken by the patient. By the same token, the pump should also be adaptable to changes in long-term patient needs and changes in ambient operating conditions.